Back in December of 2013 I heard a man on the radio say when the Bible talks about demons and fallen angels it’s the same thing. I don't claim to be an authority on this (or any other) topic but I disagree with him.
The Bible has a lot to say about angels, much less to say about demons. In fact, the word demon is not in the King James Version; it uses the words devil, devilish and devils. Those words are only used four times in the entire Old Testament, but lots of times in the New.
One of these days I'll write an article about angels, but for this one I'll be brief about who they are. Angels are heavenly beings who are superior to humans in power and intelligence. They are spiritual beings who have the ability (I assume with God's permission) to take on earthly form. Several times the Bible has them appearing on earth as a man. They seem to be organized into three functional groups: God-praising, messengers, and soldiers. The Bible doesn't say how many there are, but when Jesus was being arrested He said He could summon "more than twelve legions" of them to help Him. Even if He was talking only about Michael's soldier angels, using the average number of soldiers in a Roman legion of that day, it would be between 80 & 90 thousand. When angels are being discussed in Rev. 5:11 it talks about "10,000 times 10,000 and thousands of thousands." This expression has been estimated at between 100 million and 100 trillion. I think we can safely say there are lots of them.
The concept of fallen angels comes from Rev. 12:4, 7-9, where a great red dragon (Satan) is at war in heaven with Michael and his angel army, and v4 says the dragon "drew the third part of the stars of heaven, and did cast them to the earth." Many Bible scholars agree these stars are 1/3rd of the angels in heaven that followed Satan in his rebellion against God and were cast out of heaven. The question as to whether these "fallen angels" are the demons on earth I think was answered in 2 Pet. 2:4, where Peter says they (fallen angels) were "cast down to hell, and delivered into chains of darkness, to be reserved unto judgment." They are in prison in hades, not roaming around the earth.
So who or what are the demons and where did they come from? I think they are the spirits of the wicked "giants" destroyed in the flood of Noah's day. The word giants in Hebrew is nphiyl (nef-eel'), meaning a feller, a bully, or a tyrant. The Hebrew Bible calls them Nephilim, the same word used in ancient Ugaritic literature to describe "god-men," or half-divine demigods. It was because these Nephilim had corrupted the human race, according to Gen. 6:2, 4, that God destroyed all humans except Noah and his family. The New Testament speaks often of the spirits of devils inhabiting people and generally causing havoc on earth. Both Matt. 25:41 and Rev. 20:10 say their ultimate fate is everlasting fire (the lake of fire).